Solid state optical shutter

ABSTRACT

A method of producing an optoelectronic circuit comprising: forming a first optoelectronic element of the circuit on a first surface of a semiconductor substrate; forming a second optoelectronic element of the circuit on a second surface of the semiconductor substrate; and wherein the first and second optoelectronic elements communicate via current transmitted through the substrate.

RELATED APPLICATIONS

The present application is a divisional of U.S. patent application Ser.No. 09/402,854 filed on Jan. 3, 2000, which is a national stage of PCTApplication No. PCT/IL97/00120 filed on Apr. 8, 1997, the disclosures ofwhich are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates generally to optoelectronic devices, andspecifically to high-speed shutters for image and optical datamodulation.

BACKGROUND OF THE INVENTION

Optoelectronic shutters are well known in the art. Such shutters openand shut in response to an electrical waveform or pulse applied thereto,generally without moving mechanical parts. They are used, inter alia, inhigh-speed image capture applications, for which mechanical shutters aretypically too slow. Optoelectronic shutters known in the art includeliquid crystal shutters, electrooptical crystal shutters and gated imageintensifiers.

Liquid crystal shutters are simple and inexpensive to manufacture. Theirspeed, however, is inherently limited to about 20 μsec switching time.Moreover, in their open state, liquid crystal shutters typicallytransmit only about 40% of the light incident thereon, whereas in theirclosed state, they still transmit at least 0.1% of the incident light.

Electrooptical crystal shutters can be switched quickly, on the order of0.1 nanosecond. They require a collimated light input, however, and haveonly a narrow acceptance angle within which they can shutter incidentlight efficiently. The crystals themselves are expensive, and costly,high-speed, high-voltage electronics are also needed to switch theshutters on and off at the rated speed.

Image intensifiers generally comprise an electron tube or microchannelplate, with a photoelectric photocathode input and a light-emittingphosphor-coated anode at the output. Gated intensifiers further includehigh-speed switching circuitry, which enables them to be gated on andoff quickly, with typical switching times as fast as 1 nanosecond. Forlight to be effectively shuttered or amplified by the intensifier, itmust be focused on the photocathode. Although intensifiers aremanufactured in large quantities, the manufacturing process involvesmetal-to-glass vacuum sealing, which is complex, labor intensive andtherefore costly. Partly as a result of this complexity, gatedintensifiers tend to be large compared to their active area and areavailable in a very limited range of shapes and sizes.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a compact,high-speed, solid-state optoelectronic shutter, which may bemanufactured at relatively low cost in large quantities.

In some aspects of the present invention, the shutter is used formodulating light that is received by an image capture device, such as ahigh-speed CCD camera.

In other aspects of the present invention, the shutter is used inmodulating an image or an array of optically-encoded data, for example,in the framework of a system for optical data processing.

It is a further object of the present invention to provide a method formanufacturing the shutter.

In preferred embodiments of the present invention, an optoelectronicshutter comprises a generally planar substrate made of semiconductormaterial, having mutually substantially parallel input and outputsurfaces. A planar photodiode layer is formed on the input surface ofthe substrate, and a planar light-emitting diode (LED) layer is formedon the output surface, opposite the photodiode layer. A gate layer isformed intermediate the photodiode and LED layers, preferably adjacentthe photodiode layer. Preferably, transparent, electrically conductivecoatings, for example, indium tin oxide (ITO), are applied to at least aportion of each of the input and output surfaces. An additional biasinglayer is preferably formed intermediate the gate and LED layers, forback-biasing the LED.

When light strikes the photodiode layer, photoelectrons are created.Ordinarily, when there is no voltage or only a relatively small voltageapplied between the input and output surfaces, the electrons remain inthe photodiode layer and recombine, as they are unable to pass the gate.Under these conditions, the shutter is closed.

To open the shutter, a control voltage, preferably in the range of 5 to15 volts, is applied between the surfaces, to bias the LED positivelywith respect to the photodiode. In some embodiments of the invention,higher or lower voltages may be used. Preferably the voltage is appliedto the conductive coating on the surfaces. This voltage creates apotential difference across the substrate, between the photodiode andthe LED. In this state, photoelectrons that are produced in thephotodiode pass through the gate and substrate to the LED layer, whichemits light in response to the incident photoelectrons. This processcontinues until the control voltage is removed, whereupon the shuttercloses.

Preferably, the substrate comprises silicon, GaAs, InP or othersemiconductor material known in the art, preferably in the range of 0.5to 2 mm thick and 1 to 40 mm across. More preferably, the substratecomprises a high-electron mobility, substantially single crystal of oneof the above-mentioned materials, wherein the crystal is oriented sothat one of the crystal axes is substantially perpendicular to the inputand output surfaces. In this way, when the control voltage is on,photoelectrons emitted by the photodiode travel ballistically along thecrystal axis perpendicular to the surfaces, generally withoutsubstantial scattering and without significant photoelectron divergencein directions other than perpendicular to the surface. Hence, a photonstriking at any point on the input surface of the shutter and generatinga photoelectron in the photodiode layer there will cause a photon to beemitted by the LED layer at a corresponding point on the output surface.As a result, if an image is focused onto the input surface, it will bereproduced at the output surface with minimal blurring or distortion.

The active aperture of the shutter, defined by the areas of thephotodiode and LED, may be as large as 40 mm across and may be madecircular, square or rectangular, depending on the application. Thus,shutters in accordance with the present invention are more compact andmay have a substantially greater ratio of active aperture to thicknessthan high-speed shutters known in the art, such as gated intensifiersand electrooptical crystal shutters.

In some preferred embodiments of the present invention, the photodiodelayer comprises an avalanche photodiode. Preferably, an additionaltransparent conductive layer is interposed between the photodiode layerand the gate, and a voltage preferably of between 20 and 100 volts isapplied to this conductive layer so as to reverse-bias the avalanchediode. Each photon incident on the input surface that is absorbed by thephotodiode layer will cause an “avalanche” of electrons, to begenerated, as is known in the art. When the control voltage is applied,these electrons pass through the gate to the LED layer. Thus, shuttersaccording to these preferred embodiments transmit images with enhancedefficiency and can even provide a modicum of image intensification.

In preferred embodiments of the present invention, the shutter isproduced using methods of semiconductor device fabrication known in theart. After the substrate has been suitably cut and polished, the gate,photodiode and LED layers are preferably formed thereon by means ofepitaxy, MOCVD and/or ion implantation. Electrical leads are then bondedto appropriate locations on the shutter, specifically to the input andoutput surfaces thereof, and the shutter is suitably packaged for itsapplication.

It will be appreciated that shutters may be mass-produced in accordancewith the principles of the present invention at substantially lower costthan high-speed shutters known in the art. Shutters in accordance withpreferred embodiments of the present invention generally include only asingle, solid-state component, largely comprising low-cost,readily-available materials. Fabrication of such shutters may besubstantially automated. Shutters in accordance with preferredembodiments of the present invention may be made to operate atrelatively low voltage: typically 5-10 volts, or at most 100 volts whenan avalanche photodiode layer is used. Gated intensifiers known in theart generally require high voltage, typically at least 6,000 volts.

In some preferred embodiments of the present invention, a shutter asdescribed above is used to modulate light input to an image capturedevice, such as a CCD camera. For example, the shutter may be used inimage capture devices substantially as described in WO 98/39790, andparticularly in camera systems for range-gated and three-dimensionaldistance-responsive imaging, as described in WO 97/01111, WO 97/01112and WO 97/01113. All of these PCT patent applications are assigned tothe assignee of the present patent application, and their disclosuresare incorporated herein by reference. Light passing through the shuttermay be focused onto an image detector, such as a CCD array.Alternatively, the shutter may be directly coupled to the imagedetector, for example, by attaching the shutter to a fiber-opticfaceplate that is coupled to the detector, by fastening the shutterdirectly to the image detector surface or by using a relay lens.

In other preferred embodiments of the present invention, the shutter isused as a part of a system for optical computing. The shutter is used toswitch or modulate simultaneously a matrix of optically-encoded databits or an electronic image.

There is therefore provided, in accordance with a preferred embodimentof the invention, a solid-state optoelectronic shutter, comprising: asemiconductor material, having formed therein or thereon: a planarphotodiode, having a planar surface, and optically communicating withthe input; a planar LED layer, having a planar surface substantiallyparallel to the planar photodiode, and optically communicating with theoutput; and a planar gate layer, intermediate the planar photodiode andthe planar LED.

Preferably, the substrate comprises substantially a single crystal, andwherein an axis of the crystal is oriented in a direction substantiallyperpendicular to the planar surfaces of the photodiode and the LED.

Preferably, the semiconductor material comprises a material selectedfrom the group consisting of silicon, GaAs and InP.

In a preferred embodiment of the invention, the photodiode has a planarPIN structure. Alternatively, the photodiode comprises a planaravalanche photodiode. Preferably the shutter comprises a transparent,conductive layer between the photodiode and the gate layer, wherein anelectrical potential is applied to the conductive layer to reverse-biasthe photodiode.

Preferably, a control voltage is applied across the gate layer so as topermit electrons to flow therethrough, from the photodiode to the LED,thereby opening the shutter.

In a preferred embodiment of the invention, the semiconductor materialis comprised in a generally planar substrate having input and outputfaces. Preferably, the shutter comprises first and second transparent,conductive coatings on the input and output faces, respectively, whereinthe control voltage is applied between the first and second coatings.Preferably the shutter further includes metal coatings on peripheralportions of the input and output faces, wherein the metal coatings areelectrically coupled to the transparent, conductive coatings forapplication of the control voltage therethrough.

In a preferred embodiment of the invention, the planar photodiode isproximate the input face. Alternatively or additionally, the planar LEDis proximate the output face.

In a preferred embodiment of the invention the planar photodiode, thegate layer and the planar LED are formed proximate one of the input andoutput faces. Preferably, the substrate is thinned between the formedplanar devices and the other face.

In a preferred embodiment of the invention at least one of the faces isa face of semiconductor material epitaxially grown on the substrate andwherein at least one of the planar photodiode and the planar LED isformed in the epitaxially grown material.

There is further provided, in accordance with a preferred embodiment ofthe invention, an optical shutter comprising a photodiode which receiveslight an produces charge carriers; an LED which receives the chargecarriers and produces light responsive to the carriers; and a gate whichgates the flow of carriers from the photodiode to the LED. Preferablythe shutter includes a semiconductor filled region, situatedintermediate the photodiode and the LED, through which the chargecarriers flow.

In a preferred embodiment of the invention, the semiconductor materialcomprises a material selected from the group consisting of silicon, GaAsand InP.

In a preferred embodiment of the invention, the photodiode has a PINstructure. Alternatively, the photodiode comprises an avalanchephotodiode. Preferably the shutter comprises a transparent, conductivelayer between the photodiode and the gate layer, wherein an electricalpotential is applied to the conductive layer to reverse-bias thephotodiode.

Preferably, a control voltage is applied across the gate layer so as topermit electrons to flow therethrough, from the photodiode to the LED,thereby opening the shutter.

There is further provided, in accordance with a preferred embodiment ofthe invention, a method for producing an optoelectronic device,comprising: providing a generally planar substrate made of semiconductormaterial, having input and output faces; producing a gate layer withinthe substrate between the input and output faces; producing a planarphotodiode proximate the input face, external to the gate layer; andproducing a LED proximate the output face, external to the gate layer.

Preferably, the method comprises depositing transparent, conductivecoatings on the input and output faces. Preferably the method furtherincludes depositing metal coatings on peripheral portions of the inputand output faces, in electrical contact with the transparent, conductivecoatings thereon. Preferably, the method further includes attachingelectrical leads to the metal coatings, for applying a control voltagethereto.

In a preferred embodiment of the invention producing the gate layercomprises implanting ions in the substrate. Alternatively oradditionally, producing the photodiode comprises doping the substrate toform a planar PIN structure therein. Alternatively or additionallyproducing the photodiode comprises forming an avalanche photodiodestructure at the input face of the substrate. Preferably, the methodincludes producing a conductive layer intermediate the gate layer andthe photodiode for applying a biasing voltage to the photodiode.

In a preferred embodiment of the invention, the photodiode is formedproximate the input face. Alternatively or additionally, the LED isformed proximate the output face.

In a preferred embodiment of the invention, the photodiode, the gatelayer and the LED are formed proximate one of the input and outputfaces. Preferably, the method includes thinning the substrate betweenthe formed photodiode, gate layer and LED and the other of the input andoutput faces.

There is further provided, in accordance with a preferred embodiment ofthe invention, an optoelectronic shutter produced according to themethod described above.

There is further provided, in accordance with a preferred embodiment ofthe invention, an integrated image detection device, comprising: anoptical detector array, having a front surface; and a shutter asdescribed above, fixed to the front surface of the array so as tomodulate light incident on the array through the shutter. Preferably,the output surface of the shutter is directly attached to the frontsurface of the array. Alternatively, the device comprises a faceplate,which conveys an optical image between first and second sides thereof,wherein the first side of the faceplate is attached to the output faceof the shutter, and the second side of the faceplate is attached to thefront surface of the array.

There is further provided, in accordance with a preferred embodiment ofthe invention, a method for producing an integrated imaging device,comprising providing an optoelectronic shutter as described above andfixing the shutter to a front surface of an optical detector array.Preferably, fixing the shutter to the front surface of the detectorarray comprises cementing the shutter to the surface. Alternatively,fixing the shutter to the front surface of the detector array comprisescementing a first side of a faceplate to the surface and cementing theshutter to a second side of the faceplate, opposite the first side andoptically communicating therewith.

The present invention will be more fully understood from the followingdetailed description of the preferred embodiments thereof, takentogether with the drawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic, sectional representation of an optoelectronicshutter, in accordance with a preferred embodiment of the presentinvention;

FIG. 2 is a schematic, sectional representation of an intensifiedoptoelectronic shutter, in accordance with another preferred embodimentof the present invention;

FIG. 3 is a schematic, sectional representation of an optoelectronicshutter, in accordance with still another preferred embodiment of thepresent invention;

FIG. 4A is a schematic representation of an image detection device,incorporating the shutter of FIG. 1, in accordance with a preferredembodiment of the present invention;

FIG. 4B is a schematic representation of an image detection device,incorporating the shutter of FIG. 2, in accordance with an alternativepreferred embodiment of the present invention; and

FIG. 5 is a schematic representation of an image detection device inaccordance with a preferred embodiment of the invention, in conjunctionwith which are plotted representative curves of voltage for atransmitting and a non-transmitting state of the shutter.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Reference is now made to FIG. 1, which is a schematic, sectionalillustration showing a solid-state optoelectronic shutter 20, inaccordance with a preferred embodiment of the present invention. Shutter20 comprises a substrate 22 of semiconductor material, having an inputsurface 34 and an output surface 36. Preferably, substrate 22 comprisesa substantially pure, single crystal of GaAs, silicon, InP or othersuitable material known in the art. The crystal is oriented so that acrystal axis 42 thereof is substantially perpendicular to surfaces 34and 36.

Within substrate 22, a planar gate layer 28 is produced below inputsurface 34, preferably by implantation of ions at an appropriate rangeof depths within substrate 20, using methods of semiconductor dopingknown in the art. A planar photodiode layer 24, preferably having a PINstructure, as is known in the art, is similarly produced between inputsurface 34 and gate layer 28. A planar light-emitting diode (LED) layer26 is produced adjacent to output surface 36, opposite photodiode 24 andgate 28. An additional biasing layer (not shown in the figures), forback-biasing LED 26, is preferably produced intermediate LED layer 26and gate 28, generally adjacent the LED layer.

Input surface 34 and output surface 36 are coated with layers 30 and 32,respectively, of transparent, conductive material, preferably comprisingindium tin oxide (ITO), chemically deposited on the surfaces. Conductivemetal coatings 44 are applied to peripheral portions of surfaces 34 and36, in electrical contact with each of transparent, conductive coatings30 and 32. Electrical leads 38 and 40 are connected to coatings 44, soas to apply triggering signals to coatings 30 and 32, respectively, asdescribed below.

FIG. 5 shows a schematic voltage diagram of voltage as a function ofposition across a gate according to the invention, wherein the uppercurve represents the voltage where the device is not transmitting (e.g.where the gate voltage is positive) where the lower curve represents thesituation where the device is transmitting (e.g. where the gate isgrounded or floating).

When optical photons are incident on photodiode layer 24, photoelectronsare generated in the layer. Ordinarily, in the absence of an electricalpotential applied between leads 38 and 40, gate layer 28 forms apotential barrier, which prevents these photoelectrons from passingthrough to substrate 22. The electrons recombine within or adjacent tothe photodiode layer. In this state, shutter 20 is effectively closed,and light striking input surface 34 will be substantially prevented fromgenerating light which exits through output surface 36.

To open shutter 20, a voltage, preferably in the range 5 to 15 volts, isapplied between leads 40 and 38. Photoelectrons generated in photodiodelayer 24 are consequently able to pass the potential barrier of gatelayer 28, and are accelerated by the voltage toward LED layer 26. Whenthe electrons reach the LED layer, they recombine, whereupon photons aregenerated and emitted through output surface 36.

When the shutter is open, the electrons travel ballistically throughsubstrate 22, along a direction substantially parallel to crystal axis42, with minimal divergence or scattering. Thus, a photon incident atany point on input surface 34 will generally produce a photoelectronthat travels straight through substrate 22 and causes a photon to beemitted from a corresponding point on output surface 36. In this manner,if an optical image is focused onto the input surface, it will bereproduced at the output surface when the shutter is open. Theresolution of the reproduced image, relative to the input image, willgenerally be determined by the crystal quality and purity of substrate22, since imperfections in the crystal will cause electrons to divergeand be scattered as they pass from photodiode 24 to LED 26.

Shutter 20 may be switched rapidly between its open and shut states,with typical transition times of approximately 1 nanosecond or less.Unlike high-speed shutters known in the art, shutter 20 requires no highvoltage, and may be switched using TTL-level electrical pulses. It isfabricated using simple, generally inexpensive processes and materials,known in the art, and requires no vacuum sealing. Because of the limitedquantum efficiencies of photodiode 24 and LED 26, however, thetransmittance of shutter 20 in its open state will be low.

FIG. 2 is a schematic, sectional illustration showing an intensifiedshutter 50, in accordance with an alternative embodiment of the presentinvention, which overcomes the above-mentioned problem of lowtransmittance. Shutter 50 is substantially similar to shutter 20 in mostaspects of its construction and operation, except that shutter 50includes a planar avalanche photodiode 52 in place of photodiode layer24 in shutter 20. For each photon that it absorbs, avalanche photodiode52 generates a plurality of electrons, typically about one hundredelectrons, dependent on biasing of the diode, as described below. Theelectrons pass through gate 28 to LED layer 26 when the shutter is open,whereupon a plurality of photons are emitted by the LED. Because of thiselectron multiplication effect, the effective transmittance of shutter50 is generally close to unity, and may even be greater than unity,i.e., the shutter may intensify an image that is focused onto its inputsurface. The image transmitted by shutter 50 will typically have addednoise relative to the input image, however.

Shutter 50 is preferably produced using methods of semiconductor devicefabrication known in the art. Gate layer 28 is produced by dopingsubstrate 22 adjacent to input surface 34, preferably by ionimplantation, as described above. A transparent, conductive coating 54is deposited over surface 34, along with a metal coating 44 inelectrical contact with the transparent, conductive coating, on aperipheral portion of the surface. Avalanche photodiode layer 52 is thenepitaxially deposited over coating 54 on surface 34, as is known in theart, and outer transparent, conductive coating 30 is deposited overdiode layer 52. LED layer 26 and transparent, conductive coating 32overlaying the LED layer are produced as described above with referenceto shutter 20. Other suitable fabrication processes, as known in the artmay also be used to fabricate the device.

To operate shutter 50, a reverse biasing voltage in the range of 5 to 40volts, preferably approximately 100 volts, is applied between a pair ofleads 56 and 38, which are coupled to transparent, conducting layers 54and 30, respectively. At 100 volts reverse bias, the estimated gain ofavalanche photodiode 52 will be approximately 100 secondary electronsfor every primary photoelectron.

As long as lead 56 and lead 40, coupled to transparent, conducting layer32, are held at approximately the same potential, however, gate 28prevents the electrons from reaching LED layer 26. To open shutter 50, acontrol voltage, preferably in the range 5 to 15 volts, is appliedbetween leads 40 and 56. Under these circumstances, the electronsproduced in photodiode layer 52 cross gate 28 and reach LED layer 26,resulting in optical emission therefrom, as described above.

FIG. 3 is a schematic, sectional illustration showing a planar shutter53, in accordance with another preferred embodiment of the presentinvention. Shutter 53 is substantially similar to shutter 20, shown inFIG. 1, except that LED layer 26, gate layer 28 and photodiode layer 24are all produced adjacent to input surface 34 of substrate 22. LED layer26, at the greatest depth within substrate 22 relative to the inputsurface, is preferably produced first, followed by gate layer 28 andthen photodiode layer 24.

Whereas shutters 20 (FIG. 1) and 50 (FIG. 2) require that dopingoperations be performed at both input surface 34 and output surface 36,all the doping operations are performed in shutter 53 at the inputsurface only. Alternatively, the layers may all be produced, in reverseorder, at the output surface. Consequently, shutter 53 will be easierand less costly to manufacture than shutters 20 and 50. Furthermore,since photoelectrons emitted by photodiode layer 24 must travel only ashort distance through substrate 22 to reach LED layer 26, thedivergence and scattering of the electrons will be reduced.

Preferably, after layers 26, 28 and 24 have been produced, substrate 22is thinned, as is known in the art, so that output surface 36 is broughtclose to LED layer 26. Thinning the substrate reduces the distancebetween conductive layer 32 and LED layer 26, so that a relatively lowerbiasing voltage may be applied between leads 38 and 40. Thinning alsoreduces the attenuation of light passing through the substrate from LED26 to output surface 36.

FIG. 4A is a schematic illustration showing an integrated imagedetection device 58, comprising shutter 20, described above withreference to FIG. 1, and a CCD detector array 60, in accordance with apreferred embodiment of the present invention. Shutter 20 is opticallycemented onto front surface 66 of CCD array 60, using optical assemblymethods and materials known in the art. Shutter 20 and array 60 aremounted in an integrated circuit package 62 and, preferably, are coveredby a window 64. Electrical leads 38 and 40 of shutter 20 are coupled viapackage 62 (as are the leads of CCD array 60), as is known in the art,to receive control pulses from external circuitry. Thus, for example,device 58 may be incorporated in a CCD camera, in place of aconventional CCD detector array, without modification to the cameraoptics and with only minor changes to the camera electronics.

FIG. 4B is a schematic illustration showing another integrated imagedetection device 70, in accordance with an alternative preferredembodiment of the present invention. In device 70, intensified shutter50, described above with reference to FIG. 2, is integrated with CCDarray 60, as described above with reference to device 58 in FIG. 3. Inthis case, however, device 70 preferably includes a fiber-opticfaceplate 72, intermediate the array and the shutter and opticallycoupling therebetween, so that the CCD array is isolated from therelatively high voltage present between leads 38 and 56 of the shutter.Shutter 20 may similarly be coupled to array 60 by a faceplate, ifdesired.

Alternatively, shutter 20 or shutter 50 may be coupled to CCD array 60,or to other detector arrays and image detectors known in the art, bymeans of an imaging lens that images output surface 36 of the shutteronto the array or detector. In particular, shutter 20 or 50 may be usedin high-speed imaging applications and in range-gated andthree-dimensional distance-responsive imaging, as described in theabove-mentioned PCT patent applications, which are incorporated hereinby reference.

In some preferred embodiments of the present invention, photodiode layer24 of shutter 20 (FIG. 1) or avalanche photodiode layer 52 of shutter 50(FIG. 2) is sensitive to a radiation wavelength range other than visibleradiation, for example, infrared or ultraviolet radiation. In theseembodiments, shutter 20 or shutter 50 may be used to up- or down-convertthe radiation frequency to the visible range. Additionally, LED layer 26may be produced, as is known in the art, to emit photons at variouswavelengths, from the infrared through the visible range.

In other preferred embodiments of the present invention, shutters inaccordance with the principles of the present invention may be used inmodulating an image or an array of optically-encoded data, for example,in the framework of a system for optical data processing. Such shuttersare advantageous in optically processing the image or the encoded data,since they enable an entire array of data to be optically modulated orswitched rapidly, by application of a relatively low-voltage controlsignal, without the need for complicated or costly optical components.

It will be appreciated that the preferred embodiments described aboveare cited by way of example, and the full scope of the invention islimited only by the claims.

What is claimed is:
 1. A method of producing an optoelectronic circuitcomprising: forming a first optoelectronic element of the circuit on afirst surface of a semiconductor substrate; forming a secondoptoelectronic element of the circuit on a second surface of thesemiconductor substrate; and wherein the first and second optoelectronicelements communicate via current transmitted through the substrate.
 2. Amethod according to claim 1 wherein the semiconductor substrate is aplanar substrate.
 3. A method according to claim 1 or claim 2 whereinforming the first and second optoelectronic elements comprises dopingthe first and second surfaces respectively.
 4. A method according toclaim 1 wherein the semiconductor material of the substrate comprises amaterial selected from the group consisting of silicon, GaAs and InP. 5.An optoelectronic circuit comprising: a semiconductor substrate; a firstoptoelectronic element of a circuit located on a first surface of thesubstrate; a second optoelectronic element of a circuit located on asecond surface of the substrate; and a communication link comprising acurrent path through the substrate via which information is communicatedbetween the first and second optoelectronic elements.